In recent years, mobile telephones and portable information terminals employ a piezoelectric vibrator using quartz crystal or the like as a time source, a timing source of control signals or the like, a reference signal source, etc. As this type of piezoelectric vibrator, various ones are offered. As one of them, a surface mount device-type piezoelectric vibrator is known. As the piezoelectric vibrator of the type, generally known is a three-layer structure type one in which a piezoelectric substrate with a piezoelectric vibration member formed thereon is sandwiched between a base substrate and a lid substrate and bonded all together. In this case, the piezoelectric vibrator is housed in the cavity (sealed unit) formed between the base substrate and the lid substrate. Recently, not only the above-mentioned three-layer structure type one but also a two-layer structure type one has been developed.
The piezoelectric vibrator of the type has a two-layer structure in which the base substrate and the lid substrate are directly bonded to each other; and a piezoelectric vibration member is housed in the cavity formed between the two substrates. As compared with a three-layer structure one, the two-layer structure type piezoelectric vibrator is excellent in that it can be thinned, and is therefore favorably used. As one of such two-layer structure type piezoelectric vibrators, a piezoelectric vibrator is known, in which the piezoelectric vibration member is electrically connected to the external electrode formed on the base substrate using the electroconductive member formed to run through the base substrate (see Patent Reference 1 to Patent Reference 4).
The piezoelectric vibrator 300 comprises, as shown in FIG. 30 and FIG. 31, a base substrate 301 and a lid substrate 302 anodically-bonded to each other via a bonding film 307, and a piezoelectric vibration member 303 sealed up in the cavity C formed between the two substrates 301 and 302. The piezoelectric vibration member 303 is, for example, a tuning fork-type vibration member, and this is mounted on the upper face of the base substrate 301 via an electroconductive adhesive E in the cavity C.
The base substrate 301 and the lid substrate 302 are, for example, insulating substrates of ceramics, glass or the like. Of the two substrates 301 and 302, the base substrate 301 has through-holes 304 running through the substrate 301. The through-hole 304 is filled with an electroconductive member 305 to seal up the through-hole 304. The electroconductive member 305 is electrically connected to the outer electrode 306 formed on the lower face of the base substrate 301, and is electrically connected to the piezoelectric vibration member 303 mounted in the cavity C.    Patent Reference 1: JP-A 2001-267190    Patent Reference 2: JP-A 2007-328941    Patent Reference 3: JP-A 2002-124845    Patent Reference 4: JP-A 2006-279872
In the above-mentioned, two-layer structure type piezoelectric vibrator, the electroconductive member 305 plays important two roles of blocking the through-hole 304 to thereby airtightly seal up the cavity C, and electrically connecting the piezoelectric vibration member 303 to the external electrode 306. In particular, in case where the adhesion to the through-hole 304 is insufficient, then the airtight sealing inside the cavity C may be lost; and in case where the contact with the electroconductive adhesive E or the external electrode 306 is insufficient, then the piezoelectric vibration member 303 may work erroneously. Accordingly, for evading such failures, the electroconductive member 305 must be formed in such a state that it completely blocks the through-hole 304 while kept in firm contact with the inner face of the through-hole 304 and it has no depression on the surface thereof.
However, Patent Reference 1 to Patent Reference 4 describe formation of the electroconductive member 305 with an electroconductive paste (Ag paste, Au—Sn paste, etc.), but have no description relating to a concrete manufacturing method of how to practically form it.
In general, in case where an electroconductive paste is used, it must be fired and hardened. In other words, after the through-hole 304 is filled with an electroconductive paste, it must be fired and hardened. When fired, however, the organic matter in the electroconductive paste may be lost through evaporation; and therefore, in general, the volume after firing decreases as compared with that before firing (for example, in case where an Ag paste is used as the electroconductive paste, the volume may decrease by about 20% or so). Accordingly, even when the electroconductive member 305 is formed with an electroconductive paste, the surface may have depressions formed thereon or, in some serious cases, there may be a risk of forming through-holes in the center.
As a result, the cavity C may lose its airtightness, or there is a possibility that the electric connection between the piezoelectric vibration member 303 and the external electrode 306 may be lost.